1. Field of the Invention
The present invention relates to a delamination method of a functional thin film, particularly to a delamination method of a film or a layer each of which is provided with various elements. In addition, the present invention relates to a transferring method for pasting a separated film to a film substrate, and further relates to a semiconductor device comprising a thin film transistor (hereinafter referred to as a TFT), which is formed in accordance with the transferring method, and to a manufacturing method thereof.
2. Description of the Related Art
Recently, a technique for forming a TFT by using a semiconductor thin film (with a thickness of about several nanometers to several hundred nanometers) formed over a substrate provided with an insulating surface is attracting attention. A TFT is widely applied to an electronic device such as an IC or an electro-optic device, and is developed especially as a switching element or a driver circuit of a display device.
Such display devices can be mass-produced by performing dicing for obtaining multiple panels. Glass substrates and quartz substrates are mostly used; however, they have disadvantages of fragility and heaviness to be enlarged. Therefore, forming a TFT element on a flexible substrate typified by a flexible plastic film is being tested.
However, when a sophisticated polysilicon film is used for an active layer of a TFT; a process at a high temperature at several hundred degrees centigrade is necessary in a manufacturing process, so that the polysilicon film can not be formed directly on a plastic film.
Therefore, a method of separating a delamination layer from the substrate by using a separation layer in between is proposed. For example, a separation layer comprising such as amorphous silicon, a semiconductor, nitride ceramics, or an organic polymer is provided and exposed to a laser beam through the substrate; the substrate is separated by a delamination or the like in the separation layer (Reference 1: Japanese Patent Laid-open Publication No. 10-125929). In addition, a reference describes an example of completing a liquid crystal display device by pasting a delamination layer (referred to as a layer to be transferred) to a plastic film (Reference 2: Japanese Patent Laid-open Publication No. 10-125930). Techniques of respective companies are introduced in the articles on flexible displays (Reference 3: Nikkei Microdevices, Nikkei Business Publications, pp. 71-72, Jul. 1, 2002).
However, in the method described in the above publications, it is required to use a substrate which is highly transparent to light. Further, a rather high-energy laser beam is necessary for imparting sufficient energy to release hydrogen included in amorphous silicon through a substrate. That causes a problem of damage to a delamination layer. Further, the above publication describes a structure in which a light-resistant layer or a reflective layer is provided to prevent the damage to the delamination layer; however, in that case, it is difficult to fabricate a transmissive liquid crystal display device or a light emitting device which emits light downward. Still further, with the above method, it is difficult to separate a delamination layer having a large area.